There seems to be enough resources to level up both, so there is no harm in doing it (how it will change with more characters remains to be seen). The problem is currency, but there is awfully little of that for either strategy. At least it's relatively cheap...
And another thing is that we get fewer summons as we progress though tiers. It will be necessary to level up other characters at some point. All in all, the whole summon system needs a rework. Maybe base it on the high score rather than character levels? That sounds like a fair solution.